Organic molecular crystals have emerged as promising new materials for optoelectronic and integrated-circuit devices. Large-scale integrated circuits based on organic transistors have already been demonstrated, as described in B. Crone et al., Nature 403, 521-523 (2000). On Feb. 17, 2001 Sony announced the development of the world's largest (13″) full-color active matrix thin film transistor (TFT) organic electroluminescent display. In a series of publications in the journal Science, a team from Bell Labs (Lucent Technology) has shown that organic molecular crystals can be used in a number of high efficiency devices. For example, ambipolar field-effect transistors and inverters have been fabricated on organic molecular crystals, as described in J. H. Schön, S. Berg, Ch. Kloc, and B. Batlogg, Ambipolar Pentacene Field-Effect Transistors and Inverters,” Science 287, 1022-1023 (2000).
Optoelectronic devices such as light-emitting field-effect transistors and an organic solid-state injection laser have also been demonstrated, as described in J. H. Schön, A. Dodabalapur, Ch. Kloc, and B. Batlogg, “A Light-Emitting Field-Effect Transistor,” Science 290, 963-965 (2000), and J. H. Schön, Ch. Kloc, A. Dodabalapur, and B. Batlogg, “An Organic Solid State Injection Laser,” Science 289, 599-601 (2000), respectively. Moreover, superconducting field-effect switches can be fabricated on molecular crystals, as described in J. H. Schön, Ch. Kloc, R. C. Haddon, and B. Batlogg, “A Superconducting Field-Effect Switch,” Science 288, 656-658 (2000). The fabrication of all these devices has clearly demonstrated the great potential of organic molecular crystals for various types of integrated-circuit devices.
Currently, electronic and optoelectronic circuits are fabricated from conventional inorganic semiconductors using photolithography. High-energy electron beam lithography is used for the fabrication of the photolithographic masks. These methods incorporate spin-coating of photo-sensitive resist onto a substrate, baking the photoresist coating, exposing a pattern on the photoresist, developing the photoresist (i.e. chemically removing either the exposed or unexposed areas), performing some operation on the underlying semiconductor (e.g. doping or oxidizing) and, finally, removing the remaining resist.
Every such procedure can have a detrimental effect on device properties when an organic molecular crystal is the substrate or active layer. Organic molecular crystals are very fragile, presenting serious difficulties in processing by such conventional wet chemical lithographic methods. Up to now, either a simple low-resolution shadow-mask technology has been used for fabrication of organic devices, or all photolithography procedures for patterning of electrodes and dielectric layers are done before the active organic layer is deposited.
A conventional electron-beam lithography method, for example, may include the following steps: 1) an electron-beam sensitive polymer layer (the resist layer) is spin-coated onto a semiconductor substrate; 2) the resist layer is patterned by exposure to the electron beam, during which step the polymer chains are broken in exposed areas (for positive resist) and become soluble; and then 3) the exposed polymer layer is developed by removal of soluble exposed areas. The semiconductor material is then subsequently patterned using different etching methods (such as wet chemical etching or dry plasma etching) using the developed patterned resist layer as a mask.
Low-energy electron-beam lithography (LE-EBL) was proposed by R. F. W. Pease and co-workers (Y. W. Yau, R. F. W. Pease, A. A. Iramnanesh and K. J. Polasko, J Proc. Sci. Technol 19, 1048 (1981)), and its capabilities have been demonstrated in a number of publications, such as: Y.-H. Lee, R. Browning, N. Maluf, G. Owen, and R. F. W. Pease, “Low voltage alternative for electron-beam lithography,” J. Vac. Sci. Technol. B 10(6), 3094 (1992); P. A. Peterson, Z. J. Radzimski; S. A. Schwalm, and P. E. Russell, “Low-voltage electron beam lithography,” J. Vac. Sci. Technol. B 10(6), 3088 (1992); and T. H. P. Chang, M. G. R. Thomson, M. L. Yu, E. Kratchmer, H. S. Kim, K. Y. Lee, S. S. Rishton, and S. Zolgharnain, “Electron beam technology—SEM to micro column,” Microelectronic Engineering 32, 113 (1996).
Although LE-EBL is a promising method to reduce proximity effects in patterns with sub-micron resolution while avoiding the complicated correction algorithms used in high-energy electron-beam lithography, there is no suggestion in these references that the electron beam itself can be used to effect the removal of material from an organic molecular film. Rather, according to conventional methods, removal of material is achieved in a second etching step, with the associated problems of resolution control, contamination, and the like.